Silver-polyimide nanocomposite films: A single-stage thermally-induced metallization of aromatic fluorinated polyimides yielding highly reflective films

摘要

Highly reflective surface-metallized polyimide films were fabricated by addition of the soluble silver ion complex (hexafluoroacetylacetonato)silver(I) into a solution of a fluorinated poly(amic acid). Two fluorinated poly(amic acid)s were studied: 6FDA/4-BDAF and 6FDA/1,3(3)-APB Thermal curing of the silver(I)-doped poly(amic acid) films led to silver(I) reduction and formation of reflective surface-silvered polyimide films at silver concentrations from 2 to 10 wt% Even very low concentrations (2 wt%, 0,28 vol%) gave excellent specular reflectivity with the iso-propylidene-containing polyimides The reflective surfaces were optimized when the cure temperature was 275-300°C, although X-ray diffraction showed metallic silver as early as 200cC, After achieving a maximum specular reflectivity of 80-100% in the 275-300°C range for the silvered films, the reflectivity diminished modestly with further heating at 300°C, Starting with the soluble imide forms of 6FDA/4-BDAF and 6FDA/1,3(3)-APB gave films which were 25-40% less reflective than those beginning with the poly(amic acid). None of the reflective films was electrically conductive and could not be made so by extended heating or polishing,. The metallized films were thermally stable and maintained mechanical properties similar to those of the parent polyimide. Scanning electron microscopy revealed that the air-side surface layers of silver were nominally 40-100 nm thick; the bulk of the films contained a uniform distribution of particles much smaller than those at the surface. There was good practical adhesion of silver metal to polyimide. The 2,0 wt% silver films of non-fluorinated traditional polyimides BTDA/4,4'-ODA, BPDA/4,4'-ODA and ODPA/1,3(3)-APB were significantly less reflective than the 6FDA-based polyimides, suggesting that the presence of hexafluoroisopropylidene groups is especially effective in forming a reflective silvered surface.